1. Technical Field of the Invention
The present invention relates to a technology for generating a masking sound to prevent an original sound from being overheard.
2. Description of the Related Art
The masking effect is a phenomenon in which, when two types of sound signals having similar frequency component characteristics are propagated in the same space, it is difficult for a listener to identify the sound signals. In one technology, overhearing of spoken sound is prevented using the masking effect. In this technology, a sound signal of a vocal sound generated in a room is collected as a target sound signal and is processed into a masking sound signal having frequency characteristics which do not allow the target sound signal to be perceived as a vocal sound, and the masking sound signal is then emitted outside the room. In this case, it is difficult to hear the target sound signal outside the room due to the masking effect since both the target sound signal and the masking sound signal which has frequency components close to those of the target sound signal are emitted outside the room. Prevention of overhearing using such masking effect is described in Japanese Patent Application Publication No. 2008-233671. In a masking system described in Japanese Patent Application Publication No. 2008-233671, a target sound signal collected through a microphone in one of two adjacent rooms is divided into sections, each corresponding to one syllable, and a scrambling process is performed on the target sound signal such as to rearrange the sections of the sound signal, and the scrambled sound signal is emitted as a masking sound signal through a speaker in the other room.
However, since such a masking system simultaneously emits two types of sound signals, i.e., the target sound signal and the masking sound signal, a listener in the room may perceive noisy or unnatural sound, depending on the relation between the frequency components of the target sound signal and the frequency components of the masking sound signal.